Quick answer
A boiler is a heating appliance that heats water, or turns it into steam, and circulates it through pipes to radiators, baseboard convectors or tubing in the floor, which release the heat into each room.
Despite the name, most home hot-water boilers never boil the water. Only steam boilers do. Both burn gas or oil, or use electricity, and are rated by AFUE.
Quick facts
The key facts about Boiler, with sources:
- US homes heated mainly by steam or hot water (2020)
- 9.29 million 1
- Of those, natural gas / fuel oil
- 6.51 million / 1.62 million 1
- Federal minimum (made from 15 January 2021)
- Gas hot water 84%, gas steam 82%, oil hot water 86%, oil steam 85% AFUE 2
- ENERGY STAR
- ≥90% AFUE gas; ≥87% oil 3
- Design rule
- No constant-burning pilot on gas boilers; automatic water temperature adjustment on hot-water boilers 2
- Cooling
- None; needs a separate AC or mini-split
Key takeaways
- A boiler moves heat with water or steam through pipes, not air through ducts.
- About 8% of US homes heat mainly with a steam or hot-water system 1.
- Hot-water boilers can be condensing and very efficient; steam boilers cannot condense in the same way.
- Federal rules set 82% to 86% AFUE floors by type, and ban standing pilots on gas boilers 2.
- Boiler homes need a separate cooling system, often ductless mini-splits.
What a boiler does
A boiler is the heart of a hydronic, or water-based, heating system. It heats water with a gas or oil burner or an electric element.
A pump, called a circulator, pushes that hot water through pipes to radiators, baseboard convectors or plastic tubing in the floor. The water gives up its heat and flows back to the boiler.
Steam boilers work a little differently. They boil water, and the steam rises through pipes to radiators on its own pressure. As it cools, it condenses back to water and drains back to the boiler.
Boilers are most common in older homes in the Northeast and Midwest. Nationally, 9.29 million homes used a steam or hot-water system as their main heat in 2020. Most burned natural gas; 1.62 million burned fuel oil 1.
The hot-water loop
- Thermostat calls for heat → boiler burner fires
- Heat exchanger warms the water → circulator pump sends it into the supply pipe
- Radiators, baseboards or floor tubing release heat into rooms
- Cooler water returns → back to the boiler to be reheated
- Expansion tank absorbs the growth of water as it heats; relief valve guards against too much pressure
- Exhaust leaves through a chimney or, on sealed-combustion units, a wall vent
How a boiler works
A modern gas boiler lights its burner with electronic ignition, not a standing pilot. Federal rules ban constant-burning pilots on gas boilers 2. Hot combustion gases pass through a heat exchanger, warming the water inside it.
In a condensing boiler, the heat exchanger pulls so much heat out of the exhaust that water vapor condenses. That only happens when the water coming back from the house is cool enough.
Hot-water boilers made since 2012 must have an automatic way to adjust water temperature, such as an outdoor reset control that runs cooler water on mild days 2. Cooler water means more condensing and higher real efficiency.
Many boilers also heat domestic hot water, either through an indirect tank or, in a combi boiler, on demand.
Types of boilers
| Type | How it works | Federal AFUE floor (made from 15 Jan 2021) |
|---|---|---|
| Gas hot-water, non-condensing | Cast iron or copper exchanger, chimney vent | 84% 2 |
| Gas hot-water, condensing | Stainless or aluminum exchanger, PVC or steel vent, drain | 84% floor; ENERGY STAR 90%+ 3 |
| Gas steam | Boils water; low-pressure steam to radiators | 82% 2 |
| Oil hot-water | Oil burner, chimney vent | 86% 2 |
| Oil steam | Oil-fired steam | 85% 2 |
| Electric | Resistance elements | No AFUE floor; temperature control required 2 |
| Combi | Hot-water boiler that also makes tap hot water on demand | As gas hot-water |
Worked example: fuel use at three AFUE levels
Assume a home needs 900 therms of delivered heat per winter (an assumed load, typical of a cold-climate boiler home but not measured). Fuel burned = heat needed ÷ AFUE 2 3.
| Boiler | Math | Gas burned |
|---|---|---|
| Pre-2012 floor, 80% | 900 ÷ 0.80 | 1,125 therms |
| Current gas hot-water floor, 84% | 900 ÷ 0.84 | About 1,071 therms |
| ENERGY STAR minimum, 90% | 900 ÷ 0.90 | 1,000 therms |
| Condensing unit rated 95% | 900 ÷ 0.95 | About 947 therms |
Going from an old 80% boiler to a 95% condensing one cuts gas use by about 16% in this example. The real figure depends on return water temperature: a condensing boiler fed by hot cast-iron radiators may not condense much. Ask whether outdoor reset is set up.
Where boiler details show up
- The rating plate: input and output (often "net AHRI rating"), fuel, maximum working pressure.
- The pressure and temperature gauge on the boiler.
- The AHRI directory, which lists certified boiler AFUE 5.
- A quote: AFUE, condensing or not, venting, outdoor reset, indirect water heater, and whether the radiators were measured.
Pros and cons of boiler heat
What it does well
- Quiet, even, radiant warmth without blowing air or dust.
- Works with radiant floors, which many people find the most comfortable heat.
- Can also make domestic hot water.
- Zoning by room is simple with valves or extra circulators.
Where it falls short
- No cooling; AC needs ducts or ductless heads.
- No central air filtration.
- Slower to warm a cold house than a furnace.
- Leaks, frozen pipes and combustion risks need watching 4.
Limits to know
A boiler only heats. Adding cooling to a boiler home means either new ducts for central air or a ductless mini-split system. Many owners choose mini-splits, which can also take over some heating.
Steam systems are hard to make more efficient. They must make steam, so they cannot run at the low water temperatures that let a condensing boiler shine. Old steam systems also need balanced venting at each radiator to heat evenly.
Cost drivers, lifespan and warranty
What changes the price: input size, condensing or not, steam vs hot water, venting changes (chimney liner or new wall vent), near-boiler piping, an indirect water heater, and whether the old boiler is very heavy cast iron that must be broken up for removal.
Boilers can last a long time when water chemistry and pressure are kept right. EIA counted 23.54 million homes whose main heating equipment was 20 or more years old in 2020 1. Heat exchanger warranties are usually longer than parts warranties; read the terms.
HyreHVAC does not print boiler prices without enough local quotes. See our HVAC cost guide for what we can back.
How a boiler replacement goes
- Heat-loss calculation of the house; a radiator survey can also estimate what the emitters can deliver 6.
- Pick type, size and AFUE; plan venting and near-boiler piping.
- Permits for mechanical, gas and sometimes plumbing work; installers must hold the licenses your state requires.
- Drain the system, remove the old boiler, set the new one.
- Pipe, vent, wire, fill and purge air; set the expansion tank and relief valve.
- Combustion test, set outdoor reset curve, check for leaks.
- Final inspection.
Boiler upkeep
- Glance at the pressure gauge; ask your tech what normal reads for your system.
- Keep the area around the boiler clear and dry.
- On steam systems, a tech checks the water level and low-water cutoff.
- Bleed air from radiators if the tech has shown you how and the system allows it.
- Book a yearly combustion and safety inspection, including the flue 4.
Warning signs
- CO alarm or symptoms that fade when you leave the house: get out and call 911 or the gas utility 4.
- Water dripping from the relief valve, or pressure that keeps rising.
- Banging or gurgling pipes on a steam system.
- Cold radiators on a call for heat.
- Water pooling under the boiler or rust streaks.
- Gas smell: leave and call the gas utility from outside.
Federal boiler rules
10 CFR 430.32(e)(2). Residential boilers manufactured on and after 15 January 2021 must meet: gas-fired hot water 84% AFUE, gas-fired steam 82%, oil-fired hot water 86%, oil-fired steam 85%.
Gas boilers may not have a constant-burning pilot. Hot-water boilers need an automatic means of adjusting water temperature, except those with tankless domestic water coils 2.
Earlier floors. Boilers made from 1 September 2012 to 14 January 2021 had floors of 82%, 80%, 84% and 82% for the same four classes. Older boilers can stay in service 2.
ENERGY STAR. 90% AFUE or more for gas boilers and 87% or more for oil 3.
Boiler vs related systems
| System | Heat carrier | Cools | Best fit |
|---|---|---|---|
| Boiler | Water or steam | No | Homes with radiators or radiant floors |
| Furnace system | Air | With AC | Homes with ducts |
| Air-to-air heat pump | Air | Yes | All-electric homes |
| Air-to-water heat pump | Water | Some models | Low-temperature radiant systems |
Common misconceptions
- Myth A boiler boils water.
- Reality Only steam boilers do. Hot-water boilers heat water well below boiling.
- Myth A condensing boiler always runs at its rated AFUE.
- Reality It condenses only when return water is cool enough; hot radiators can keep it from doing so.
- Myth Boilers are obsolete.
- Reality Federal rules still set new boiler floors, and ENERGY STAR lists boilers at 90%+ 2 3.
- Myth You can add central air to a boiler.
- Reality Not through the boiler. Cooling needs ducts or ductless heads.
Where boilers matter most
Boilers cluster in older, colder housing, such as northern Ohio and much of the Northeast, where long winters give efficient boilers more hours to save fuel. In warm states like Texas or Arizona, boilers are rare and a heat pump usually wins. Heating degree days show how much your climate leans on heat.
When a boiler is the right call
If your home already has radiators or radiant floors in good shape, replacing the boiler is usually simpler than switching to ducts. Pick a condensing hot-water boiler where your emitters can run cooler water.
If you also want cooling, price a boiler plus ductless heads against a heat pump that covers both. Our best mini-splits page and the heat pump comparison tool help frame that choice.
Questions about Boiler
What is the difference between a boiler and a water heater?
A boiler heats water to warm the house through radiators or floors. A water heater heats water for taps and showers. Some boilers do both, either with an indirect tank or as a combi boiler.
Is a boiler better than a furnace?
Neither is better in every home. A boiler gives quiet radiant heat but no cooling or air filtration. A furnace heats fast and shares ducts with central air. The right pick usually depends on what distribution your house already has.
What AFUE should a new boiler have?
At least the federal floor: 84% for gas hot-water boilers made since 15 January 2021. ENERGY STAR requires 90% or more for gas boilers and 87% for oil. Condensing hot-water boilers go higher when return water runs cool.
Do boilers need maintenance?
Yes. A yearly inspection checks combustion, safety controls, venting, pressure and leaks. EPA recommends yearly inspection of central heating systems, including flues and chimneys, to prevent carbon monoxide problems.
Can a boiler explode?
Modern home boilers have pressure relief valves and other controls to prevent dangerous pressure. A relief valve that drips or a gauge reading far above normal is a reason to call a technician right away. Never cap or plug a relief valve.
What is a combi boiler?
A combi boiler is a hot-water boiler that also heats tap water on demand, with no storage tank. It saves space but can limit how many showers run at once. It is common in smaller homes.
Can I add air conditioning to a home with a boiler?
Yes, but not through the boiler. Options are new ducts with a central AC or heat pump, small-duct high-velocity systems, or ductless mini-splits, which many boiler homes choose because they need no ducts.
Why do my radiators bang?
Banging in steam systems usually means water trapped in pipes that should drain back, often from sagging pipes or wrong boiler water level. In hot-water systems, trapped air can gurgle. A technician should diagnose it.
What is outdoor reset on a boiler?
Outdoor reset is a control that lowers boiler water temperature on mild days and raises it in cold weather. Cooler water lets a condensing boiler condense more and waste less heat. Hot-water boilers made since 2012 must have some automatic temperature adjustment.
Sources
- U.S. EIA, 2020 RECS Table HC6.1, Space heating in U.S. homes, by housing unit type (final data March 2023), retrieved .
- eCFR, 10 CFR 430.32(e)(2), Boilers (version of 1 September 2026), retrieved .
- ENERGY STAR, Boilers, retrieved .
- U.S. EPA, Carbon Monoxide's Impact on Indoor Air Quality, retrieved .
- AHRI Directory of Certified Product Performance, retrieved .
- ACCA, Manual J Residential Load Calculation (8th Edition), retrieved .
Expert review
Written by the HyreHVAC Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: AFUE floors, dates and design rules copied from the eCFR text of 10 CFR 430.32(e)(2); ENERGY STAR tiers from its boilers page; counts from EIA RECS 2020 HC6.1, all read 10 October 2026. The fuel example divides an assumed load by AFUE.
Suggest a correction. We fix errors and say what changed.